This research is concerned with relationships between neuroelectric measures of human brain function, as manifested by brain event-related potentials (ERPs) recorded from scalp, and psychological measures of cognitive behavior. We are specifically concerned with a group of long latency, positive polarity ERP components, referred to as the late positive complex (LPC), that relate to endogenous central nervous system activity involved in processing and evaluation of information. The various components of the LPC may occur together and may overlap. The relationships of these components to experimental variables appear to differ from one another. Our objective is to contribute to the understanding of at least three of these endogenous components: (1) A long latency, parietal component, commonly referred to as P300 or P3b, which relates systematically to evaluation and categorization of information extracted from the eliciting event; (2) Slow Wave -- A long duration potential that appears to relate to processing demands and mobilization of effort; (3) P300E -- A relatively short latency, frontal component which may also relate (but not in the same way as P3b) to information extracted from the eliciting event. The approach is to establish systematic relationships between observable differences in ERP components and changes in experimental conditions. This involves two types of strategies: (1) To crosscut in the same experiment independent variables which presumably influence these components in different ways: (2) To use long intervals between stimuli in order to reduce orverlap among components. (2) To use long intervals between stimuli in order to reduce overlap among components. The independent variables manipulated will include perceptual requirements, conceptual requirements, degree of informational redundancy and level of difficulty. The proposed investigations are intended to broaden the range of human neuroelectric phenomena that can be used as noninvasive tools for obtaining information on the physiology of mental operations and that can contribute to diagnosis of neurological and psychiatric disorders.